1. Field of the Invention
This invention relates to a television synchronous deflection circuit, and more specifically to such a circuit for securing a stable operation when power is initially supplied.
2. Description of the Related Art
A typical conventional television receiver is generally comprised of a tuner for selecting a desired channel signal among VHF and UHF television signals received via an antenna and for converting the selected channel signal into an intermediate-frequency signal, a picture intermediate-frequency amplifier for amplifying the picture intermediate-frequency signal, a detector for detecting the amplified picture signal outputted from the picture intermediate-frequency amplifier and for outputting a composite picture signal, a synchronous separator for separating a horizontal synchronous signal from the output signal of the detector, a horizontal deflector for generating a horizontal deflection signal which is to be applied to the horizontal deflecting coil of a cathode-ray tube and which is based on the horizontal synchronous signal separated by the synchronous separator, a color signal processing circuit for generating the necessary three primary color signals (i.e. red, green and blue) from color picture signals, as well as other signal processing circuits.
In the horizontal deflector, there is provided a horizontal AFC (Automatic Frequency Control) circuit for eliminating noise pulses in the horizontal synchronous signal outputted from the synchronous separator and for preventing any shift in horizontal synchronization.
The horizontal deflector is operable upon receipt of the D.C. current obtained from a commercial power supply (e.g., 50 Hx/100 V) by diode rectification.
Meanwhile, the circuit blocks such as the picture intermediate-frequency amplifier, detector, synchronous separator and color signal processing circuit are operable upon receipt of the D.C. current from a second power supply obtained by smoothing a high-frequency flyback pulse obtained from a flyback transformer for the generation of an anode voltage for the cathode-ray tube.
Generally, in this conventional television receiver, after the first power supply is turned on by a main switch, the second power supply is built up. But since the quantity of charging electric charges of a capacitor in the synchronous separator is small before the output voltage of the second power supply reaches a predetermined voltage, a charging current from the second power supply flows to the capacitor, and as a result, an abnormal signal (i.e. not a correct horizontal synchronous signal) would be outputted from the synchronous separator. Therefore the horizontal AFC circuit would output an abnormal frequency signal missing the safe operating region of a backward-stage horizontal drive transistor. When an abnormal signal is applied to the horizontal AFC circuit while a charging current flows to the capacitor, an abnormal control voltage will be applied to VCO in the horizontal AFC circuit. Thus, an oscillation frequency will fluctuate extensively in the horizontal AFC circuit. In such a case, the VCO may undergo harmonic oscillation (e.g., at a quadruple frequency). Then, a signal whose frequency is higher than the normal one (f=15.734 Khz) would be applied to the horizontal drive transistor. Normally, the horizontal drive transistor has its load coupled to a coil, at which a high voltage flyback pulse is generated. While the flyback pulse is present, the horizontal drive transistor remains off. In other words, turning off the horizontal drive transistor issues the flyback pulse. If a high frequency is present under this condition, the transistor which remains off would be turned on. Therefore, the horizontal drive transistor would be damaged by a large current caused by the high voltage flyback pulse.